Laminating With Less
Using Adhesives to Reduce Material and Energy Usage in Flexible Packaging
By Michael Reed, president of Morchem
As flexible packaging has become capable of replacing thicker, more rigid structures by meeting the increasing protective, security and brand demands of today’s packages, it is an unfortunate reality that many of these flexible packages are not widely recycled. Apart from recycling facility limitations, one of the biggest reasons for this is that many of these packaging structures are multilayer and multi-material. Mechanical recycling is still the prevalent recovery technology and it is difficult to use this to recycle structures comprised of different materials such as foil, nylon and polyester – all have different behaviors such as densities and melting points, which severely limit sorting and separation.
This reality is leading the industry away from the improbability of mixed plastic recycling to the much more feasible recyclability of monomaterial structures. These are easy to laminate and easy to recycle. Polyolefins like polyethylene and polypropylene are widely recycled and extremely versatile, and consequently are preferred monomaterial substrates, but they do not have the properties on their own to satisfy the requirements of today’s flexible packaging applications, which often require high performance like extended shelf life.
There are ways to extend the functionality of the adhesives and coatings that hold these packages together. Recent innovations in chemistry enable the use of certain polyurethanes and polyesters to reduce material and energy usage in the creation of the flexible package they also hold together, while also improving package recyclability or material reuse – chemical multitasking! This improves overall package sustainability and often decreases the cost of the package in the process … do I have your attention now?
The adhesive lamination process is typically fairly energy intensive, with three of the highest energy users being: 1) evaporation of the solvents in solvent-based adhesives; 2) creation of in-process scrap; and 3) the laminate curing process. Let’s look at adhesive solutions that can deliver reductions in each of these areas:
Evaporation of the solvents in solvent-based adhesives
Although the performance of solvent-free adhesives continues to improve, solvent-based adhesives are still most often used where more demanding chemical and thermal package requirements exist. During the manufacture of the film and after bonding of the layers, the remaining solvent must be evaporated in hot ovens, with the corresponding energy to do so. By increasing the level of solids in the solvent-based adhesive system while maintaining performance, the solvent is reduced, as is the work that has to be done and the energy that has to be used in this step.
This also means lower volatile organic carbons (VOCs) and a lower carbon footprint. This reduction can be significant at over 15% of the solvent being replaced with actual adhesive or solid. Higher solid content translates to more actual adhesive in every delivery, which reduces the number of shipments needed, further lowering carbon footprint. This also reduces the amount of flammable inventory that must be held onsite at the film manufacturer. If you needed further convincing, eliminating solvent also reduces overall system cost.
Creation of in-process scrap
In-process production scrap uses a lot of energy in its creation, and it’s frequently seen in film lamination because manufacturers are trying to maximize efficiency and run product as fast as they can to meet demand, short lead times and just-in-time delivery. Common polyester/ polyethylene laminates and metallized or specialty barrier films often experience carbon dioxide bubble formation in the external layer due to the solvent-free system chemistry and high production speeds.
Over time, the weight of the roll of film can remove these inside the roll where the pressure is applied, but a manufacturer can lose many yards of film on the outside of the roll where there is no weight to force the carbon dioxide out. Improvements in the aromatic / aliphatic ratio of the polyurethane chemistry reduced carbon dioxide generation in the adhesive system, which means the film manufacturer no longer depends on the weight of the roll itself to remove carbon dioxide, and they’re not throwing away thousands of feet of film in the process.
This can also translate to a significantly (30%+) increased laminator throughput that is no longer limited by conventional bubble formation. It also typically means that the reels can be released faster for slitting, which removes another production bottleneck. The combination of reduced energy, improved yield and increased output achieves an impressive consolidated sustainability improvement and cost savings.
The laminate curing process
The curing of film laminates typically uses a hot room to accelerate the process, making film available faster for shipment. These hot room are energy intensive and so become a savings opportunity for adhesive innovations. Retort packaging is perhaps the most demanding and intensive, with a very long cure time and energy requirement. It is fairly standard to use retortable pouches with a foil barrier, but the foil barrier traditionally places limitations on the type of adhesive that can be used.
Aromatic systems provide high chemical resistance, but they cannot be used in retort applications like these foil barrier pouches as they generate primary aromatic amines (PAAs) that are limited by food regulations. These PAAs must be able to exit the package during curing, and foil prevents that, meaning the PAAs could leach into the packaged food. So aliphatic (non-PAA) systems must be used, but they come with a very long cure time and energy requirement.
Modifying the polyurethane chemistry of the system enables acceleration of cure, in some cases by over 70%, while maintaining the required high chemical resistance. The manufacturers can not only slit and pouch faster, but see a significant energy savings – in this example, five days of cure room energy. It also means that production sites without hot rooms can cure at room temperature and enter markets that were previously not accessible to them.
Innovations in adhesive chemistry
Beyond energy reductions, material usage in these flexible packages can also be reduced with innovations in the adhesive chemistry. Heavier film gauges and exotic materials such as ethylene vinyl alcohol (EVOH) or nylon are often used to provide the oxygen and moisture barrier properties required of the package, limiting rework, recycling and reuse. Synergistic adhesive/barrier systems can be created to reduce or eliminate these exotic materials, allowing for easier recycling and downgauging of film structures.
EVOH is a typical material used in the industry to provide oxygen barrier in flexible packaging. This requires a converter to inventory many different SKUs of film: varying structures and thicknesses of EVOH and substrates like polyethylene. This translates to supply chain complexity and risk, with EVOH shortages having occurred over the last few years. By deploying an oxygen barrier/adhesive system, the converter becomes master of their own destiny, by simply laminating the type of structure they need when they need it. These oxygen barriers are designed in conjunction with MORCHEM solvent-free adhesive, so that there is no compromise to the final bond strength of the laminate, while decreasing oxygen transmission rate (OTR) to levels approaching that of EVOH.
With the replacement and elimination of EVOH in these structures, the move into monomaterials and increased recyclability becomes possible, as the level of adhesive and barrier being used is well below the thresholds established by organizations like Recyclass and APR.
However, as triplex lamination and film downgauging become more prevalent due to their improved efficiency and reduced cost, these thresholds will be challenged and further developments in adhesive chemistry are required to keep pace.
This is where the development of fully recyclable adhesives provides a corresponding partner to the oriented polyolefin grades created to improve film appearance and thermal qualities. Such adhesives are usable above the Recyclass and APR thresholds when they have been tested by those organizations and found to have no impact on the properties of the recycled structure. Such performance also enables true package circularity, rather than downcycling into lower value applications like trash bags.
Even though they represent the smallest percentage of the package, these innovative adhesives and coatings show even more promise: they will continue to hold packages together while imparting new levels of mechanical, chemical and thermal performance – levels that enable significant reductions in energy and material used to create those packages…an answer to the question posed by the increasing challenge of sustainability initiatives around the world.
About the Author
Michael Reed is President of Morchem’s North American business and a chemical engineer, with over 25 years of experience leading and growing plastics and packaging businesses around the world. Learn more at www.morchem.com.